The invention relates to a method and apparatus employing elemental zinc for the purification of aqueous solutions of metallic salts (especially aqueous solutions of zinc sulfate) from copper and cadmium present in the solution.
In certain well-known techniques of this type, the purification is carried out by the reaction, on copper and cadmium salts in the solution, of zinc dust which is continually stirred in a container having a volume of 50-100 m.sup.3. The reaction causes the copper and cadmium in the solution to solidify together in the form of a cake, which can then be removed, e.g., by filtration.
This method has been found to be disadvantageous in that the production of the required zinc dust is complicated and involves irreversible metallic losses. Moreover, the amount of zinc dust required is several times higher than the stoichiometric amount, and in any event is extremely high when a substantially complete purification (e.g., below 0.2 mgr/l) of the solution from the contained copper and cadmium is to be achieved. Additionally, in such method the purification step is lengthy, leading to the redissolution of the solidifed copper and cadmium in the purified solution.
In another well-known method of this type, the purification is accomplished with the use of a pulsating, densely packed column of zinc granules. The pulsating mechanism is conical in shape and typically exhibits a pulse amplitude of 1.5-3 cm and a frequency of 180-200 impulses per minute.
The second method is also disadvantageous in that the pulsations cause only insignificant displacements of the finely packed zinc granules, so that the resulting friction forces arising from the displacements of the granules are not strong enough to effect a complete separation of the copper-cadmium cake formed on the surface of the granules from the granular surface, so that the cake tends to stick to the granules. Accordingly, the cake grows more dense toward the surface of the granules and gradually disrupts the required contact between the solution and the zinc. As this phenomenon proceeds, the total reaction surface area of the granules is gradually reduced, and the unseparated cake remaining in the reaction chamber continually plugs the small pores between the granules. As a result, the reaction between the zinc and the remaining copper and cadmium salts in the solution becomes continually less effective.